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Does the EASA LAPL(A) have a FAA equivalent?
I have an EASA LAPL (A) fixed wing pilots license and would like to know what the equivalent FAA license would be.

I'm not sure about their equivalence (meaning one cannot fly in US with a LAPL right way), but the closest that comes is the Recreational Pilot license.
The requirements are roughly similar:

30 hours flying experience; however, the FAA requires 3 hours of solo flying and cross country requirements are slightly different.

The limitations are also roughly similar:

Recreational pilot license doesn't allow the pilot to have more than one passenger, while the LAPL does (upto three).

The new EASA LAPL licenses (A, S, etc.) are sub-ICAO level licenses, meaning that they do not conform to ICAO standards (required minimum training hours, for one).
Since ICAO is normally the framework under which various countries accept each others' licenses, there is currently no recognition of LAPL outside of EASA member states.


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Why would aircraft take-off in the opposite direction as landing despite wind direction?
I have visited and flown out of Harare International Airport, Zimbabwe a number of times and the thing that always baffles me is: Why do aircraft use runway 23 to depart, but runway 05 to land?
At first I've thought that it may be a wind change, but it appears it's not the case. Is it because wind is limited at the airport and therefore direction does not matter? Or is there some special procedure for 'hot/high' airports?

Related if not duplicate: Do planes ever intentionally takeoff or land downwind?

Apparently, only the 05 runway has precision approach markings and (Medium Intensity) Approach Lighting System (MALS). Runway 23 has non-precision approach markings and no Approach Light System.
So, the aircraft only land on runway 05.

This is a great answer - I'm wondering what the limits would be to cause aircraft to be unable to land 05 considering it can be a tailwind landing. Different for each aircraft due to weight and landing distance? Or would the officials stop using it at a certain limit?

The limit will be different for each aircraft, but typically around 15 kts. This does mean that aircraft will sometimes be unable to land on 05. The aircraft will take off into the wind where possible regardless of which runway is being used for landing, as the headwind gives a little extra safety margin.

You’re both right - aircraft have certified limits which aren’t permitted to be exceeded (15 kts is, as mentioned, an often used value), but additionally landing performance (weight, speed, runway state and associated stopping distance) must be considered, too, and could lead to an even more restrictive operational limit under many conditions.

Wind direction does matter! Just to make that clear from the start.
Harare Airport is located at an elevation of 1490m MSL, and Zimbabwe is in tropical region, which classify the Airport in hot and high conditions. That's also why Harare has one of the longest runways in Africa.
As I understand it, pilots are required to follow standard procedures, but on some specific conditions, they are able to submit a request for special ones (or suggested by controllers). To list a few :

start take off roll from runway entry instead of from one end of runway (to save time when late on schedule, to ease traffic flow, to avoid a runway backtrack when no taxiway leads to runway start while an aircraft is on an approach pattern, etc.)
downwind take off (similar to above, or last take off(s) after a wind change, etc.)
take off from the runway that shortens flightplan length/duration (there are specific paths you must follow (waypoints) to exit the area of an airport, and reach the air routes toward destination. There are also forbidden areas like cities or military/restricted zones you have to avoid). Obviously, taking off direction matters.
on a calm day with clear visibility and nearly no wind, on an airport with not that much traffic, you have much more opportunity to select a preferred runway depending on the flightplan.

There are also some cases where you can land from one runway end and take off from the other. I don't know whether Harare suits those cases, but there are several airports in the world which are. Obviously the ones that have closed runway on one end like Lukla (VNLK) or Courchevel, but you also have Nosy Be (FMNN) and Antsiranana (FMNA) in Madagascar which aren't, to name those I know well, and another one that is closed: Kai Tak (VHHH) in Hong Kong. Reasons vary:

Terrain. When you have mountains right in the take off path.
Winds, likely to come from one side along the year, that you can neglect on landing on calm days, but not on take off.
Lack of Navigation support from one end of the runway (ILS/PAPI/VASI/VOR/DME...)
Restricted area (City, Military bases, highly secured facilities, etc.)
Gates/parking spots location: when they are near the end of one runway, take offs are likely to start there, and landing to occur from the other end, when possible.
noise abatement : landings can be tolerated from one end, but not take offs.
etc.

Now, sum the above for Harare:

you have such a long runway that sometimes, not all aircraft are required to take off from one end. Reciprocally, you have to taxi longer to reach Rwy23 for take of, and taxi back longer to reach the terminal if you landed on Rwy05.
you have many airlines operating there, but there is not that much activity as on platforms like JFK, Manchester, Singapore Changi or Sydney.
you have airliners, commuters, general aviation and military; large, medium and small aircraft. Depending on their flightplans and the weather condition, you can expect to have take offs and landings from both directions, anytime.
you're in Africa, where rules are less strict than anywhere else (good or bad ? I don't know)

So, when winds are calm, visibility clear enough, runway long enough for a given aircraft to take off from a specific point, you can expect pilots to select the best runway that fits their flightplans. This does not happen only in Harare, it happens on a daily basis, like on my home airport as well, Antananarivo Ivato (FMMI/TNR):
Due to winds direction mostly coming from east, take off are expected to occur from Rwy11. But, small airliners like ATR or some 737/ERJs doesn't backtrack Rwy11 at all. Only "visitors" backtrack the runway. Medevac small aircraft (Cessna 206B, Learjet 31, King Air) usually land from the runway that suits their flightplans. A few minutes later, a DHC6 takes off downwind (Rwy29) because the destination is to the west. Then a 777F of Air France lands on Rwy11. And so on while the terminal is right in the middle between the two ends of the runway, and it's only 3100m long.

By the way, pilots or controllers don't just do what they want. Every decision is taken with the uttermost professionalism and care, and whenever that is required, this is registered in log books, or a clearance is requested, and granted beforehand. That means almost all, if not all, activity you may spot on Harare are known and defined procedures. It's highly unlikely a pilot would perform a procedure not written anywhere (the agency/organization in charge of navigation safety in the country is responsible for providing standard procedures).

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Why (and when) did people start meowing on guard frequency?
meow
Some love it, some hate it, but we've all heard it - that's right, pilots meowing on guard frequency (121.5).
When did it start? Does (or did) it represent something?
Does ATC ever do it?

Don't know if that is better or worse than "guuuuuaaaaaarrrrrrrrrrddddddddddd"...

ATC comms are recorded at the individual control position. The tapes are occasionally pulled for random auditing. Unlike a lot of other decisions a controller could make, meowing on guard isn't really defensible...

@RonBeyer - Yes, but we know when that originated - October 22, 2105:  https://youtu.be/YoZE0nE60sk?t=232

2105? Were they time travelers?

To be followed by "Who dat?", "Who dat who say who dat?"...

We've all heard it?  Not so.  Indeed, I don't really know what you mean by "on guard".

@james, you are familiar with the "guard frequency" 121.5, I hope? It is common for pilots to make a transmission on guard unintentionally, upon which they will receive the reply "ON GUARD" from a multitude of airline pilots. Or, if they're unlucky, a pilot impersonate whoever the first guy tried to contact. Usually this happens to airline pilots but GA folks may also mistake guard for CTAF, or so I gather from the responses I overhear at my facility. When I transmit on guard, I will say "N12345, Podunk Approach on guard," and so will other pilots if they are using guard intentionally.

This problem seems to be specific to CONUS (which may also be a clue toward its origins), so not everyone will have heard it.

@randomhead: I've never seen 121.5 called "guard frequency", it's always been emergency or distress frequency.

@jamesqf https://aviation.stackexchange.com/q/887/31936

Do I look like a cat to you boy? Am I jumpin' around all nimbly bimbly from tree to tree? Am I drinking milk from a saucer? DO YOU SEE ME EATING MICE?

https://www.aopa.org/news-and-media/all-news/2017/july/pilot/turbine-on-guard

https://youtu.be/eMDknmovgP4?t=265

https://aviation.meta.stackexchange.com/a/3179/42636 point #11

“Meow” is a reference to a well known movie trope from the US mindless 2001 comedy from the improvisational comedy troupe, Broken Lizard Production, called Super Troopers. In the film, the main characters are Vermont State Highway Patrol law enforcement officers who play games and pranks to break up the monotony of patrolling a very rural area. The same trope is repeated in the 2018 sequel, Super Troopers 2.
After the movie premiered, law enforcement officers would often mimic the characters in the movie amongst themselves. It is extremely rare that a police officer would do this with the general public. It would be a matter of professionalism.
I would hazard a guess that ATC controllers conduct themselves according to a similar professional standard and decorum. Any utterance of “meow” on frequency is more likely to come from a General Aviation pilot.
The Cat Game

I would +1 for the origin if you had stopped there, but the article linked by @DeltaLima suggests it is airline rather than GA pilots.

I postulate that the ATC includes, just as any cohort, a wide range of personal traits. There are asshats among them too.

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Turbine engine part identification
I picked this up at a yard sale (we call them boot fairs in the UK)
Can anyone identify it ? I'm not good with gas turbines. It is stamped with the serial number 579451A. Its 40 inches in diameter, and the blades/vanes are approx. 12 inches long. There are 51 blades in total. I think it's made by Pratt and Whitney

Any help appreciated

These vanes are probably the vanes of some fan/compressor, Googling the part number without the "A" and vanes gives you links to spare parts providers hinting to second stage stator vanes.


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Why does the Pilatus PC-24 have such a large "Airplane Support"?
The PC-24 has, under the fuselage, a kind of huge support to hold the wings.

If we compare with the Citation CJ4 or the Phenom 300, this difference is quite significant. Why??

What makes you think that bulge is support structure to hold the wings? Generally in aircraft, the wings hold the fuselage rather than the other way round, but I don't think that's the purpose of this feature either.

@pericynthion I have no idea about structures, so probably is the other way around as you say

It could be a wing main spar carry-through (effectively the wing holding the other wing, albeit for bending loads), or luggage space, or a place for the main gear wheels to go (PC-24 has larger wheels than most similar sized jets), or an aerodynamic fairing to reduce interference drag... I'm not posting an answer because I don't know which of these (if any) it is. But it's not necessarily structural.

It's not a wing support, it's a plane support.  ;)

The paint scheme there really accentuates the heck out of it.  It might work better to select a picture that has a paint scheme comparable to the other pictures.

To add to the other answer...
The Pilatus PC-24 is billed as a business jet that can operate out of rough airfields. Comparing the pictures of the PC-24 to the Phenom 300 or CJ4, you can see that there is quite a bit of extra ground clearance. You can also see that the rear landing gear on the PC-24 are much more robust (dual wheels, longer travel length) than the other two aircraft.
If you look at this picture:

Source: Wikipedia
You can see that the wheels also retract into that area. They probably wanted to keep the wing low to take advantage of ground-effect to improve short-field performance. So having a higher-up fuselage allowed them to put the engines up higher and avoid FOD damage, while the low wing allowed them an impressive short field take-off performance. The PC-24 can operate out of a field just 2,800 feet long. The Phenom 300 takes 3,200 feet and the CJ4 takes about 3,500.
So the "bump" is there to be able to handle additional loads, elevate the fuselage, and store the gear. I can't find any diagrams of the fuel system, but it's possible that it also houses fuel tanks or baggage.

Also I have seen that the interior in the pc-24 is flat, while the cj4 and the phenom have a step. Coult it be related? To have more space in the interior or something like that

The huge support, or bulge, is a fairing, designed for reduction of wing root drag.  So many people talk about wing tip vortex drag, but much more is created at the wing/fuselage interface, especially in uncoordinated flight. This improvement, first seen in 1930s vintage gliders (where else!), improves the airflow around the aircraft, resulting in significant fuel saving.
Importantly, this is a subsonic effect.  Many super sonic aircraft actually reduce fuselage diameter near the wing to reduce drag, complying with "Area Rule".

Another reason for this fairing is to provide space for the main wheels to retract. The cabin needs to be circular to hold pressure and the wing isn't thick enough. I don't know why it should be different between these particular aircraft though...

They might have some fuel/cargo in there too.  Overall, very practical design, keeping wings nice and thin.  Possible hatchling of airliner.

Transsonic transport aircraft are usually area-ruled to some extent, but in their case the biggest increase in cross-section is the engines, and the wing root fairing shape tends to be shaped to match—including here, the wing root fairing ends just before the engines.

In that area, the wing spar is interfacing with the fuselage, and the retractable landing gear needs space.
In larger airliners, that space is largely inside the cylindrical fuselage, in the level underneath the passenger floor, and the cabin is still high enough to walk in the aisle.
The cross section of the business jets is usually quite small, so having the spar crossing the cabin would make a bump in the cabin floor. In order to not reduce cabin height, that structural space gets pushed "slightly" outside of the cylindrical fuselage body.
That said, other large airliners also have a large bulge - check the A380 for example.

edit:
I did some more digging into that.. Let's focus on a comparison of the PC24 and the CJ4.
I didn't find any reliable drawings or models, so while the bulge certainly seems bigger in the pictures,  it's hard to tell how much bigger it really is. Also - designs are always a compromise, and we do not know all the boundary conditions Pilatus or Cessna had, so we can only make educated guesses as to why the bulge is larger on one than on the other.
In order to explain different designs, best look at the differences in mission profile. A big difference between the CJ4 and the PC24 I know of is that the PC24 has "rough field" certification. And here we can start to dig..

The CJ4 has a "single wheel" main landing gear (one left, one right), which retract into the wing, so it doesn't need any space in the bulgy fairing.
The PC24 has twin wheels, and they do retract into that bulgy fairing, likely because they are too wide to fit into the wing. The wing also seems to have a more elaborate flap system, likely leaving even less space there for a retracting landing gear. I would attribute both to the rough field certification:

you want larger / softer .. wheels with more footprint on soft soil
while I unfortunately didn't find comparable data for landing speed / take off speed / stall speed, I assume that the more complicated flap system is there to allow for slower landing/takeoff speeds of the PC24, while maintaining cruise speed.

Concerning the aerodynamics: the bulge doesn't strike me as particularly aerodynamic, and if it was, it shouldn't be a differentiating factor between the PC24 and the CJ4. Cruise speeds between the two are similar (Wikipedia: 440kn for the PC24 vs 451kn for the CJ4), assuming that cruise aerodynamic efficiencies are also similar, both planes should converge there in the design. Assuming similar efficiencies there is valid because good aerodynamic knowledge for cruise operating points is around for longer than both planes exist.
So .. Where do they differ? The PC24 as the younger plane might have profited from newer fluid dynamics methods, CFD, turbulence & separation prediction. If anything - these improvements won't be so visible during cruise, but maybe improve handling at slow speeds, stall characteristics etc. It would strike me as odd though, if the bulge improves stall characteristics.
Bottom line for aerodynamics: I strongly doubt that the bulge is larger in the PC24 for aerodynamic reasons.
Finally, another example to support my point: the Hawker 800 has also twin wheels on the main landing gear, and it also has a relatively large bulge:

Note: all pictures are from Wikipedia.

This is the correct answer. The crossover of a low wing with a narrow radius fuselage leaves bulgy stick-out failing bits, unless the wings are relatively thin.

@Koyovis: this is the correct answer to why in general that area is big on business jets. But the question is why on the PC-24 that are is bigger than comparable airplanes. The correct answer is actually this one.

@sophit I think this is even more so correct answer why the wing root fairing is bigger in PC-24—as mentioned in the other comment, most other bizjets do have a step in the cabin over the main wing spar, PC-24 does not.

@JanHudec: hm, the Citation CJ4 (which is in the question) has no step over the wing spar and has anyway a smaller bulge than PC-24

@sophit my guess is that in that case the difference is aerodynamic—CJ4 is slower, with straight wings, so it can do with sharper transition.

@JanHudec: then you agree that it's more an aerodynamic reasons than a structural one (?)

@sophit, it's a combination of both. It is high for structural reasons and then aerodynamic reasons force it to also be long.

@JanHudec, sure but then again the question is why on the PC-24 that bulge is bigger than on CJ4.

yeah .. you guys compelled me to inflate my answer a bit.

just realized lots of it was mentioned by Ron Beyer already..

@CarlBerger: well, now it has become a very good answer  Anyway RonBeyer says that that bigger bulge is to keep the fuselage far from the field but if it is bigger then it is actually closer to ground...


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Why does the SSM decoding vary between different A429 label types
For A429 there are different label types, for example: BCD, BNR, Discrete...
These types have their corresponding SSM (Sign/Status Matrix) that is unique, for example an SSM of "00" represents a failure for BNR but represents a normal operation for a Discrete.
Could someone explain why is that the case? Why not just have a the same SSM interpretation for both Discrete and BNR?
I looked online for an answer but to no avail.
Thank you


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Why do glider licences have fewer restrictions than powered aircraft?
Under FAA rules a license for gliders doesn't require a medical certification and you can get your license at 16 rather than 17 for powered aircraft. You can get your student license two years sooner at age 14.
It appears to me that gliding would require MORE skill and experience than a powered aircraft. It looks very unforgiving. You only have one shot at landing, there is no go-around. If you lose too much altitude you could end up not being able to reach the airport. You have to know how to find thermals, etc to gain altitude. While being towed you will be flying in closer proximity to another aircraft than you normally would in a powered airplane.
Why are the restrictions less for a glider?

For starters, this article - http://flighttraining.aopa.org/magazine/2002/May/200205_Features_Gliding_Your_Way_To_A_Pilot_Certificate.html - on AOPA should be interesting.

@RaajTram Great article. But it does reinforce my feeling that gliding is more challenging than powered flight. The article explains how people who learn by gliding have a better appreciation for things that powered pilots often treasure for granted. Although there are fewer things to think about, namely engine and proper controls, it seems altogether much more challenging.

Riding a bicycle requires more skills that driving a car. However, you need no driving license for bicycles. The conclusion is that the difficulty in obtaining a license is not directly connected to the level of skills required for driving a certain vehicle but with the danger you pose to others and yourself if you drive that vehicle.

@Energizer777 I had thought of the bicycle/car analogy myself, but the dangers involved seem radically different, whereas the difference between the dangers of gliders and planes are pretty close to the same. You can still die in a glider crash; you could still cause a mid-air collision; if you crashed into a house or something you'd still do a lot of damage. After reading Peter's answer, though, I see that gliders do move a lot more slowly so those are a lot less of a risk than I was realizing (having never flown either).

The FAA makes rules based on the effects to paying passengers and innocent bystanders. They don't worry a bit about the pilot.  Gliders have much reduced energy in them, therefor less damage upon striking the head of an innocent bystander.

First of all, gliders fly slowly. Everything happens more slowly than in other aircraft, and this helps to make flying them less challenging than you might think. Flying them well is a different matter - this requires a lot of skill and good physical health.
Historically, the early age limits were introduced in Nazi Germany when the Deutscher Luftstportverband (DLSV) and later the NSFK organized groups of adolescents to let them build and fly gliders. Flight training started at the age of 14, mostly in very simple, bungee-launched gliders like the Zögling or SG-38 which were flown in summer and built or repaired in Winter. Since bungee launching is a very labor-intensive process and flights last only a few seconds, this was less about pilot training than keeping the young occupied and under the influence of the Nazi party.

Bungee launch of a Zögling glider (picture source)
And it worked. Accidents did happen, but were almost never fatal since all operations happened at low speed and altitude. Therefore, these age limits were exported together with the idea of flying unpowered aircraft.
Note that in Europe the same health certificate is required for glider flying as for flying GA aircraft.
Now to your concerns about the demanding nature of gliding:

Glider flying is done from airfields which are suitable for GA aircraft (or the gliders could not be aerotowed) or winch launching. 800 meters is about the minimum, and this leaves plenty of space to land a glider.
Flight training is done initially only in the direct vicinity of the airfield, and students are supposed to stay within a cone which allows them to reach the airfield at any moment. Flying cross-country comes much later, when the students have made more than a hundred landings and know how to get the plane down in a certain spot. Again, the slow speed helps to make such landings simple if the pilot is prepared.
Today, training is initially in two-seaters, so there is an experienced pilot around for the first landings and the first cross-country flights.
Aerotowing is also quite simple, and altitude management while in tow is more challenging than keeping the right distance from the towplane. Again, this is done initially in two-seaters, and all the glider pilot has to do is to fly with the same bank angle as and slightly above the towplane. The rope takes care of the rest.

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What is the requirement of a higher glidepath/approach angle and is this a flight safety issue?
To reduce noise in the London urban area, Heathrow airport has announced plans to trial a steeper approach for aircraft landing - changing from 3 to 3.2 degrees. If this is successful, they will further steepen the landing approach to 3.5 degrees. Moreover, London City airport uses a 5.5 degree landing approach.
What are the requirements of higher angle landing approaches and how do they affect flight safety?

The most important sources of noise in an aircraft (especially during landing) are the engines, the slats/flaps and the gear. An aircraft’s angle of descent has an effect on the noise experienced by people below. Steeper the angle, the less time an aircraft spends at low altitudes, which means that fewer people should be affected by higher levels of noise.
The reduction in noise (as experienced in Frankfurt airport) due to steeper (3.2$^{\circ}$) landing approaches are due to the following:

The aircraft is higher (compared to the 3$^{\circ}$ landing angle), and as a result, less noise is heard by the people below.
The deployment of flaps before intercept may also be 0.7 nm closer to the airport as the ILS intercept occurs 0.7 nm closer to the airport.
Thrust increase for a/c stabilization 0.3 nm closer to the airport; this results in lower noise. The thrust required was also reportedly lower.
Landing gear deployment is 0.4 nm closer to the airport (at the same height of 2000 ft)

The differences due to changing the glideslope to 3.2$^{\circ}$from the present 3$^{\circ}$ is shown in the following image:

Image from Experience with the steeper approach angle of 3.2 degrees - Presentation by DLR
As for safety, this system is expected to pose no problems as most aircraft and pilots are easily capable of doing this. As an added measure, the trial is only being carried out when visibility is good (Cat I) and the trial is optional (for aircraft using ILS anyway).
Some airports (like London City) have very steep landing approaches for clearing obstructions and may require special certifications and aircraft modifications.

good answer, but could benefit from mentioning some of the repercussions of a steeper approach. A steeper approach means descending faster, which in turn causes an increase in speed. On a 3.2 degree approach, this isn't too much of an issue. Aircraft typically use a small amount of thrust on approach anyway, thrust can be reduced at 3.2 degrees, allowing a further reduction in noise. On a 5 degree approach, this increase in speed becomes a problem. Some aircraft need a special "LCY" button fitted, which causes the spoilers to partially deploy to counteract the increase in speed.

@GavinCoates, no, faster descend does not cause increase in speed. It causes increase in power that needs to be compensated by reducing engine power or adding drag.

@JanHudec in what form does this increase in power manifest itself? Speed, surely?

@GavinCoates, if the pilot does not reduce engine power to compensate, then yes. But the pilot (or A/T) does reduce engine power to compensate so the speed remains the same, because the pilot needs the speed to be the same.

@JanHudec but that's what I just said in my comment! On a 5 degree approach, reducing engine to idle is not enough to bring the speed down - you need to deploy spoilers to counter the increase in speed.

@GavinCoates, which aircraft is certified for steep approach and flies it with spoilers all the way down to landing?

@JanHudec I said partially deployed. The A318 requires partial deployment of spoilers. See http://www.dream-air.ru/new/pilotam/13_steep_approach.pdf

@GavinCoates, it does not matter whether partially or fully (according to that document it is fully, by the way), but whether all the way to touch down (yes, it appears to be the case).

Since the forward speed is determined by required lift, steeper approach path means higher vertical speed.
Considering approach speed of 130 knots:

at 3.0°, which is 5.2% or 1:19.1, the vertical speed is 690 ft/min,
at 3.2°, which is 5.6% or 1:17.3, the vertical speed is 736 ft/min,
at 3.5°, which is 6.1% or 1:16.3, the vertical speed is 805 ft/min and
at 5.5°, which is 9.6% or 1:10.4, the vertical speed is 1268 ft/min.

Since normally the criteria for stabilized approach is that vertical speed does not exceed 1000 ft/min, the 5.5° requires different procedure with different criteria.
Vertical speed equals power ($P = mgv_v$ where $m$ is mass, $g$ is gravitational acceleration and $v_v$ is vertical speed) and to maintain speed, the engines have to develop less power by this amount, or drag has to be increased.
This is usually not a problem for propeller aircraft, especially those with variable pitch propellers (which turboprop requires and larger piston-engined aircraft have as well). With maximum RPM selected the propeller produces quite a lot of drag at idle, but since everything is already spinning fast, adding power has immediate effect if needed to go around.
It may, however, be problem for jet aircraft. Jet engines need to spool up to increase thrust and from very low power settings that can take several seconds.
The normal glide-slope is well below the gliding angle with no power (with flaps and gear), so the engines are still running at considerable power. In the slightly steeper approaches the power is less, but the difference is not as big, so it usually isn't a problem.
But the 5.5° steep approach is getting close to the unpowered glide angle even with full flaps and gear and is steeper than unpowered glide angle with lower flaps settings. This means the engines will react more slowly and pilots have to consider it. Also in some aircraft an engine-out approach can't be flown at full flaps, because the remaining engine(s) would not have enough power for go-around at that drag, which means such aircraft can't fly steep approaches with failed engine. And some aircraft may not be permitted to do these steep approaches at all, because even at full flaps the engines would run too slow and take too long to spool up.

That last paragraph does a great job of addressing the "Is it safe?" question.

What is the requirement of higher angle landing approach?

Is basically requiring the aircrafts to stay at higher altitudes for longer and approaching the ground faster. The normally used term is "glideslope".

how does it affect flight safety?

The airport authorities will have performed a safety assestment and decided that this change would not sensibly affect flight safety. What will change is how the pilots will have to perform the flare manoeuver (a bit more sharply than before) and for unexperienced students might lead to a higher rate of hard landings, but I doubt that there are many student pilots landing with airliners at those airports.


text:
Which refers to CFI renewal based on pass rate, "activities" or "duties and responsibilities"?
It's that time again, gotta re-up my CFI.
I always forget when I'm filling out IACRA whether CFI renewals (in the United States) based on student pass rate percentage are considered "activites" or "duties and responsibilities".
Which of the two, "duties and responsibilites" or "activities" is the appropriate choice when submitting an IACRA application for a CFI renewal based on pass rate?

You want to check the Activities box.
The phrasing is a little weird, and it certainly doesn't line up with the regs (see 14 CFR §61.197), but that's what you want:

(2) Submitting a completed and signed application with the FAA and satisfactorily completing one of the following renewal requirements—
(i) A record of training students showing that, during the preceding 24 calendar months, the flight instructor has endorsed at least 5 students for a practical test for a certificate or rating and at least 80 percent of those students passed that test on the first attempt.
(ii) A record showing that, within the preceding 24 calendar months, the flight instructor has served as a company check pilot, chief flight instructor, company check airman, or flight instructor in a part 121 or part 135 operation, or in a position involving the regular evaluation of pilots.
[...]

So, best as I can tell, that means:

§61.197(2)(i) is activity - 80% pass rate on 5+ students in the preceding 24 calendar months.
§61.197(2)(ii) is duties and responsibilities - serving in a position that involves standardization flights or stage checks.

Disclaimer: unfortunately, all of the AOPA material on CFI renewal is designed to get you to buy their FIRC (eyeroll), and the unhelpful FAA IACRA document for instructors says squat as well. I'm having a hard time finding a reference that proves my interpretation, but I know it's correct!


text:
How to calculate aerodynamic efficiency at minimum power conditions?
The drag polar of a propeller driven aircraft is
$C_{D}=0.039+0.071 C_L^2$.
The lift curve slope of the wing ($C_{L_\alpha}$) is 0.09/deg and zero lift angle ($\alpha_{L=0})= -1.5^o$.
Stall angle of the wing is $15^o$.
I am trying to find out the aerodynamic efficiency (L/D). Also if it even can fly with such specifications? If it can, what could be the angle of attack?

How are you finding the Design of Fixed Wing UAV course ?

At cruise condition, a propeller propulsor can be modeled as: $P=TV$, with constant output power as a function of airspeed. Your objective is to find the cruise condition such that the output power is minimized.
Assuming you know apriori what altitude you want to cruise, then density $\rho$ is determined. At level flight:
$$T=\frac{P}{V}=(C_{D_0}+KC_L^2)\frac{1}{2}\rho V^2S$$
At cruise, weight is equal to lift. At minimum power, we have $\frac{\partial P}{\partial V}=0$:
$$\frac{\partial P}{\partial V}=\frac{3}{2}C_{D_0}\rho V^2S-\frac{KW^2}{\frac{1}{2}\rho SV^2}=0$$
Do some algebra, and we have:
$$3C_{D_0}=KC_L^2=C_{D_i}$$
Therefore, the minimum power condition is not equal to maximum L/D (as an aside, it is equivalent to the condition for maximum rate of climb, which is pretty intuitive).
Now you can find the corresponding $C_L$ and verify whether it's within the valid range of the lift curve. Generally, we want at least 1.3G margin to stall warning or stall buffet. So a 1.4G margin to $C_{L_{max}}$ might not be a bad place to start.

To determine lift coefficient we have the stall angle and zero lift angle. At minimum power condition we calculate at (L/D) maximum. I am finding it hard to go with the answer. Can you create the equations as such.

@ShishirMaurya See modified answer. Remember, your OP is asking about minimum power condition of a propeller propulsed aircraft, not max L/D.

That really helped. Thanks.


text:
What does the German AZF radio license test comprise of in the oral examination?
I understand you're required to have an AZF-license, on top of a BZF-license to do IFR radio transmissions in Germany; I also figure the test is similar to the BZF license with a written and an oral part.
What's a typical sequence of radio calls in the oral test? Are there any particular gotchas one should be aware of?
I've read through a list of phrases (out of something called NfL), so I'm vaguely familiar with the differences from US phraseology, but I got from the BZF test there are other differences, like you tend to get a question "Are you ready to copy", rather than the "advise ready to copy" I'm used to.

There are a lot of differences in the US and European communication.
I got my AZF about a year ago and basically you have to do simulated IFR departure and approach. In addition you will have to fill out an IFR flightplan and answer 30 IFR communication questions.
I would say it is a similar difficulty like the BZF, it might be even a little bit easier, as you just have to repeat what the controller tells you (but you still have to be aware where you are).
An example on how radio communication is done can be found at http://www.caa.co.uk/docs/33/CAP413Supplement.pdf or http://training.ivao.de/trainingsunterlagen.html?tx_drwiki_pi1%5Bkeyword%5D=IFR-Funkbeispiel
I would however recommend a training at a flight school (which costs around 150 Euros).

Training at a flight school is absolutely recommended. It is true that the AZF is a lot easier then the BZF exam, but you should be aware of what's coming up and that's not done by reading but by practice.
If the controller is in a good mood on your exam day you may have spent a little to much money on preparation, but if they had no breakfast that morning you are happy to be used for copying clearances you will never hear when you are sitting in a plane and no longer at a table.


text:
What is the proper procedure for leaving class D airspace?
So one thing that always struck me as kind of odd was when you depart a class D tower (VFR) and you get your departure instructions, that's kind of ... it. You just go on your merry way and switch off frequency whenever you want to.
Do people just stay on the tower frequency listening until they're outside of the airspace? Or do you give the tower a courtesy call (N123 leaving the class D) when you get to the boundary, and then switch off?
Or, is it legal to switch immediately to another frequency as soon as the tower says 'left turnout approved, resume own navigation'? It always seemed a bit of a hole to me. But, are controllers bothered by a extra "check out" on leaving the class D?

I have added the [tag:faa-regulations] tag. I assume you are not interested in an ICAO/EASA based answer?

The AIM covers this from an operations / legal standpoint:

3-2-5. Class D Airspace

4. Departures from:

(a) A primary or satellite airport with an operating control tower. Two‐way radio communications must be established and maintained with the control tower, and thereafter as instructed by ATC while operating in the Class D airspace. [emphasis mine]

While operating in Class D airspace, you must maintain two-way radio contact with the tower – monitor the frequency and be prepared to respond.
You may leave the frequency once you are either clear of the airspace, or receive instructions to that end from ATC ("N1234, frequency change approved").
Whether or not controllers are bothered by you letting them know that you've left, I can't say, but it would probably qualify as frequency congestion. If they want to get in touch with you, they'll try ("N1234, are you still with me?"), but once you're outside the Class D airspace they're generally only providing advisory information.

Good answer; just wanted to add that the actual reg covering this is 91.129, paragraph c. The AIM in and of itself is not a regulatory document, but it does serve as the official guide to a good many items of interest, including how to abide by the regulations laid out in 14 CFR as exemplified by the section quoted above.

You are responsible for staying on frequency as long as you are in the class D space, UNLESS you are granted an early frequency change.
Once you leave Class D, you are free to change frequencies without notice.
If its a busy area, many controllers will still give you traffic advisories as long as you're nearby, and its reasonable to stay on frequency until you're considerably outside their space.
If its a very quiet area, it might be friendly to tell the controller to have a nice day as you're leaving.
One reason you may want an early frequency change is if you want to talk to another authority ASAP.
For example, Boeing Field (KBFI) and Renton Municiple (KRNT) are very close together, with adjoining Class-D spaces.  Its common for practice flights to go between them.
(Example map).
The conversation may go like:
You: "Boeing Tower; N513WF: Holding short, Rwy 13L, ready to go; Request early Frequency Change to Renton"
Tower: "3WF: Cleared for Takeoff, Rwy 13L.  Frequency Change approved above 400 feet.  No turns until instructed."
You get to 400+ feet, then call up Renton:
You: "Renton Tower: N513WF, out of Boeing's 13L, request touch-n-gos"
Renton: "3WF: Enter Right-traffic, following a Warrior, you're #2"
Congrats:  You're talking to Renton, while still inside Boeing's Delta-space.

If its a busy area, many controllers will still give you traffic advisories as long as you're nearby, and its reasonable to stay on frequency until you're considerably outside their space. <-- That's an important point. Generally I think it's a good idea to continue monitoring tower (or CTAF/UNICOM) until you're well outside the range where inbound aircraft may be calling up, as an aid to situational awareness. (Or until you're talking to some other ATC facility, which generally serves the same purpose.)


text:
Why do some pilots call a go-around an 'overshoot'?
A couple of months ago I was watching some Ice Pilots NWT re-runs, and I noticed they kept saying 'overshoot' when talking about a go-around. Is this standard Canadian phraseology? Or is it perhaps old phraseology? Or is it simply reworded for the benefit of the TV audience?
I'm not asking about the meaning of the word 'overshoot' which is quite clear, but it was being used were every pilot I know would use the word 'go-around', i.e. calling "overshoot, overshoot, overshoot" when shoving the throttles forward.

because overshooting the touch-down limit is the most common reason for a go-around maybe?

@ratchetfreak while that might be true, given the context, I'm fairly certain they called the action an 'overshoot' rather than a 'go-around'.

I've never heard that terminology, but a quick Google search suggests it can indeed mean "go-around" in British, Australian and (presumably) Canadian aviation lingo.

Looking at the Glossary for Pilots and Air Traffic Services Personnel (TP 11958E) at the Government of Canada web site, the definitions of overshoot and go-around are

overshoot
(1) To pass beyond the limit of the runway or landing field when
  trying to land.
Fr: dépasser
(2) Other expression for: go-around

and

“Go around”
An expression used in radiocommunications to instruct a pilot to
  abandon an approach or landing.
Fr: « Remettez les gaz »
go-around
The procedure followed by a pilot who decides to abandon an approach
  or landing.
Fr: remise des gaz

Plus, looking at various Flight Test Guides listed at the Canadian Government Air Transportation site it appears that pilots in Canadian flight schools are taught "overshoot" procedures for missed approaches (vice go-around).
Seeing as you were watching a reality TV show dealing with Canadian pilots, it is not surprising they use the terminology they were most likely taught in flight school (in Canada).

Cool, thanks for digging that up!

the quotes around "bush pilots" meaning you don't consider them real bush pilots? (just curious about your punctuation there. ;)

Overshoot could be, also, when the pilot is not able to intercept the localizer, due to a sharp bank, or faster than usual, missing to intercept it. e.g.: The crew overshot the localizer, then they decided to start a new approach procedure.


text:
What is the purpose of squawking 7777?
Aircraft use transponders to communicate to ATC their position and status. Some squawk codes are reserved, such as 7700 (emergency), 7600 (communication failure), 7500 (hijacking), 1202 (glider), 1200 (VFR), etc. One of these, 7777, is apparently used for "military interception." What does this mean in the United States? Under what circumstances would it be used on a civilian / military aircraft?

In countries outside of the US, 7777 may be used by test transponders (RABMs) to check correctness of radar stations (BITE). e.g. on top of a mountain.
In the US, it seems that it is used as well on active air defense missions without ATC clearance. This would mean that the interceptor aircraft would change it's squawk to 7777 for the military/civilian air traffic controller to see it properly (if not filtered out on civil radars).
A link to the US intercept procedures is here.

Yep, I have been wrong for about 10 years now.  I would have sworn the craft being intercepted was the squawker of 7777. Edited to add my link to the FAA doc. Deleted my wrong answer, and upvoted this.

@RyanMortensen good thing you didn't get intercepted in the last 10 years :)

@pericynthion yeah, I may need to have a chat with some people I have learned it wrong from and spread the knowledge.

You know, now that I think about it, I bet I was told that by retired Air Force guys who mixed up the roles' codes.

@RyanMortensen That's slightly concerning..

According to the AIM 4-1-20(e):

Under no circumstances should a pilot of a civil aircraft operate the
  transponder on Code 7777. This code is reserved for military
  interceptor operations.

The ATC orders don't add anything useful and a lot of security procedures are classified, or at least not publicly available on faa.gov. But it seems from that information that a) 7777 is important for interception operations, and b) civilian pilots must not use 7777. That implies that 7777 is reserved for interceptor aircraft, not the aircraft being intercepted.
The FAA's interception instructions for pilots say that intercepted aircraft should squawk 7700.
See this question too.

Under normal circumstance even interception missions are supposed to work with ATC.  Squawking 7777 is sort of a last resort thing for the DoD to tell ATC to get out of their way and get everything else out of their way.
If you listen to the Washington Center ATC records from 9-11, there's a point where the interceptors around the capital get ordered to squawk quad-seven because Washington Center was putting them into patterns NORAD didn't want them on.

Squawking code 7777 will cause the acronym "AFIO" to flash next to the target symbol on an ATC scope. This is often used when the interceptors are not talking to ATC. It helps us keep other aircraft clear of the operation.
This functionality has been around for a long while, as I'd seen it before I retired from ATC in 2012. (I also use it in developing simulator scenarios)
The code is also automatically observed and recorded by other entities.
Here's one example of their usage:
https://www.oredigger61.org/?cat=44

This adds nothing to existing answers that are over five years old.

Sure, it does. It adds what we actually see on the scope, which we couldn't divulge 5 years ago.

It is for if an aircraft is being intercepted by the military, to let ATC know so they can take the appropriate measures

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.


text:
What is the purpose of VFR-on-top?
VFR-on-top is an IFR clearance in the US that a pilot can request to allow an IFR flight to use VFR altitudes provided that it can remain in VMC. Requesting and receiving a VFR-on-top clearance doesn't cancel the IFR flight plan.
When is this clearance useful? It allows the pilot a greater choice of cruising altitudes, but why would a pilot prefer to use VFR-on-top rather than request a different IFR altitude? VFR-on-top adds at least one task to the pilot's workload - maintaining VFR cloud clearance - without reducing the IFR workload.

The VFR on top clearance is often given with a block of altitudes.  Such as this from the reference document "Maintain VFR-on-top at or between six thousand and one zero thousand.”  With this clearance the pilot can fly any altitude they want within this block of altitudes without having to contact ATC for further clearance.  This flexibility can be useful with the conditions are such that the clouds come and go as you fly along.  So if you are cruising along at 6500 and encounter a cloud and need to climb to 8500 to maintain clearance you just do it.  No need to contact ATC and wait for them to ensure they can clear you higher while the clouds keeps getting bigger and bigger in the windshield.
Even if there are not cloud issues, the ability to change altitudes may be useful in seeking an altitude with more favorable winds.

Being in VMC has several benefits over hard IMC, especially if you don't have an autopilot, so being on top has advantages. :

Manual flying with visual reference to the horizon is much easier and less strain than manually flying using instruments for attitude control
Being out of cloud enables a pilot to see and avoid traffic
Flying in cloud can be bumpy, VFR on top is often much smoother
Icing is a hazard when flying in cloud, being on top of them means you won't ice up

VFR on top works well when you have a stratus layer that has an even top, not so well when you have clouds at differing levels, so it's not always a viable option.

Those are benefits of VMC, not VFR.

In a training environment, VFR-on-top is fantastic.  You can use your IFR clearance to punch through a cloud layer to clear skies on top, practice maneuvers (or what have you), and then shoot an approach back down.  VFR-on-top can turn a bad weather day into a training day, and anytime you can do that (and get real-world actual instrument experience on top of it) is a good day.

being cleared VFR on top doesn't automatically allow you to deviate from your route

@Steve V.:  I think you mean a To VFR on Top clearance where you climb IFR and then cancel IFR once above the clouds.  In these situation the pilot is usually given a climbing vector or departure procedure to get on top of the clouds.  There is no full route clearance in a To VFR on Top clearance.

An unaddressed benefit of going VFR-on-top is the ability to file a flight plan beyond navaid limitations. ATC can issue the clearance, and not worry about distance criteria.
An example would be a flight requesting KBFF..KFSD at FL090. Before GNSS and ADSB, I'd give you three choices for such a route:

KBFF.V169.RAP.V26.PIR..FSD..KFSD. (Adds 75 miles to the flight)
A climb to FL130 for radar coverage. (Aircraft may not be capable, long term)
An explanation that you could go direct if you requested VFR-on-top.

Without explaining ALL of the options, I would not say anything about VFR-on-top, because I cannot solicit it. If the options are explained, it is not considered solicitation.
Also, some issues in the above answers need to be addressed.
"The VFR on top clearance is often given with a block of altitudes. Such as this from the reference document "Maintain VFR-on-top at or between six thousand and one zero thousand.” With this clearance the pilot can fly any altitude they want within this block of altitudes without having to contact ATC for further clearance."
This excerpt is from the instructions for holding separation, and should not be used in a clearance with enroute flight. That's because the pilot is responsible for separation from other aircraft. The ability to climb/descend to any appropriate VFR cruising altitude is inherent in the VFR-on-top clearance.
Lastly, as fjch1997 pointed out, being cleared VFR on top doesn't automatically allow you to deviate from your route.
"When an aircraft has been cleared to maintain “VFR‐on‐top,” the pilot is responsible to fly at an appropriate VFR altitude, comply with VFR visibility and distance from cloud criteria, and to be vigilant so as to see and avoid other aircraft. The pilot is also responsible to comply with instrument flight rules applicable to the flight (e.g., adherence to ATC clearances)."


text:
How does this RC helicopter keep itself upright?
I have this (cheap, beginner-level) RC helicopter:

It has 3 controls: one for climbing and descending (collective throttle), one for yawing (differential throttle), and one for pitching (the tail rotor control). There's no roll control.
This helicopter seems to have a very strong tendency to stay upright. Even if you grab it by the skids in mid-air and tilt it slightly, it will right itself (after first flying in whichever direction you tilted it in).
As shown in the picture, the helicopter has two coaxial main rotors and one tail rotor. The tail rotor is pointed vertically, so that it produces a pitching moment. The lower main rotor is fixed-pitch, but the upper rotor has cyclic pitch controlled by a weighted "balance bar". The balance bar itself is mounted about 45° ahead of the rotor. The balance bar is on a hinge so that the ends can move up and down relative to the shaft. If one end of the balance bar goes up, then the blade closer to it is automatically set to a coarser pitch; meanwhile, as the other end goes down, the blade closer to that end is set to a finer pitch.
It seems very unlikely that this helicopter has any electronic accelerometers or gyroscopes.
So, how does this helicopter keep itself upright? Here's what I can figure out myself:

Suppose that the fuselage accidentally rolls to the right while the balance bar remains upright. Then the rotor's cyclic pitch will be set so that each blade is coarsest when it's in the forward right position, and finest when it's in the rear left position. This will produce a left rolling moment, which will tend to bring the helicopter upright again. (It will also produce an up pitching moment... or maybe a down pitching moment, thanks to phase lag? Or no pitching moment at all? I don't know.)
Suppose that the fuselage and the balance bar both accidentally roll to the right. This will cause the helicopter to fly to the right... which will somehow cause it to right itself? But I don't understand the details of why this will happen.

By the way, I've noticed that the helicopter has a tendency to fly in clockwise circles, especially after being disturbed. (It doesn't yaw during this circular motion; it simply moves in a circle while maintaining a constant heading.) I bet that this tendency is caused by the balance bar somehow, but I don't know how.
(Someone may be tempted to answer, "It rights itself because the rotors are above the center of gravity." That explanation doesn't work, though, because the only way an aircraft can right itself is by means of torque. The rotors will generate this torque somehow, but they won't generate it by virtue of being located above the center of gravity.)

As a side note, it is likely that this model has electronic yaw gyro. It wouldn't be able to keep heading that well without it. Yaw is the only axis that the bar can't help with. Next time, when hovering, try to twist the body to change the heading, and you'll feel (and hear!) resistance.

@Zeus Well, it doesn't keep heading that well; you have to manually set a yaw trim wheel, and even after you do, the heading drifts more or less quickly. Next time I fly it, I'll try yawing it by grabbing it with my hand, and I'll see what happens.

As at least one answer points out, it's gyro stabilized by the balance bar. Acting as a gyroscope, the bar probably precesses slowly, which would manifest in the rotor thrust vector doing the same, which would result in it flying slow circles.

Re last paragraph-- what about the simple toy which is a propeller fixed to a dowel rod?  Rub your palms together and it flies in the air-- no tendency tip over-- what is going on there?

The top rotor is a hinge offset rotor with a very serious stabiliser bar. These rotors exert torque via the mechanism in item 1 in this answer, and the body will align itself with the top rotor. But the other way around as well: the rotor aligns itself with the shaft, it just depends on what it controlled, the rotor angle (like in a regular helicopter through cyclic pitch) or the body angle.
So top rotor and shaft will return to be perpendicular to each other after a disturbance or a control input. Torques exerted by the body are instantaneous, torques exerted by the top rotor have a time delay due to the inertia in the stabiliser bar.
The helicopter is flown by body tilt.

Pitch direction: body tilts, upper rotor follows, in a controlled way, resulting in longitudinal movement.
Roll direction: no control input possible. Once there is lateral movement, the helicopter can right itself if the aerodynamic drag on the rotor assembly is larger than drag on the body - if the other way around, the helicopter will speed up and tilt itself more and more until it crashes.

Notice that the pendulum fallacy does not apply to helicopters: they can align rotor thrust away from the CoG, like a hang glider does when canting the wing, and create a rolling or pitching moment that way.
On the flight in a circle without changing yaw (with the helicopter flying backwards halfway in the circle): thanks to @ZeissIkon in a comment:

The "flies in circles without changing heading after being disturbed" behavior is most likely due to precession of the balance bar. Disturb the fuselage/rotor shaft, some of that disturbance propagates into the balance bar; once the body has righted, the balance bar continues in a very slightly tilted plane, and the slight righting force from the shaft causes it to precess. – Zeiss Ikon

Hmmmm, interesting. I don't think the top rotor teeters, though. The rotor blades are attached to the shaft by a joint which allows the blades to advance and retreat, but not to move up and down. (There is some play in the joint allowing the blades to move up and down, but if one blade moves up, that doesn't cause the other blade to move down.) The stabilizer bar does teeter, of course. If I grab the helicopter while it's flying, and tilt it, the rotors move instantly (as if the rotor disc were rigidly attached), but the stabilizer bar lags behind by a second.

Ah so the stabiliser bar is not connected to the upper rotor? And the upper rotor blades can flap and lead/lag? Does deflecting the stabiliser change the blade pitch of the upper rotor?

Well, the stabilizer bar is connected to the upper rotor through a linkage. Deflecting the stabilizer does change the blade pitch of the upper rotor (that's what the linkage does). When one end of the stabilizer bar goes up, the rotor blade next to it (45 degrees behind it) is tilted into a coarser pitch (so it produces more lift); meanwhile, the other end of the stabilizer bar goes down, and the rotor blade next to it (again 45 degrees behind) is tilted into a finer pitch.

I'm not totally clear on what rotor blade flapping is and how it works, but I'd say that the rotor blades are capable of moving up and down by, perhaps, 5 degrees. (It seems like this is not by design, but maybe it is.) The blades are free to lead and lag; they seem to have about 90 degrees of freedom to do this. By the way, since the blades are attached to the hinge at the leading edge, both blades will try to tilt into a finer pitch; the blade which is already at a coarser pitch will exert a stronger force than the other blade.

By the way, I should clarify that the "tendency to fly in clockwise circles" is not a tendency to yaw; it's a tendency to move in clockwise circles while maintaining a constant heading. I'll edit my question to clarify that.

OK so it is a hinge offset rotor then. They can apply torque on the mast.

centre of lift will align itself above the centre of gravity if left undisturbed This is what is confusing me about this answer. Once you hold it tilted for a few seconds the center of lift is aligned with CoG, just not vertically. I'm trying to understand how it aligns itself vertically. All my attempts at reasoning fall afoul of the pendulum fallacy. I can't seem to identify the source of any rolling moment.

And if the body experiences drag the rolling moment would be in the opposite direction that to right itself vertically.

@0xDBFB7 I just took a quick peek at that study, but the difference I see is that the mechanism shifts CoG out of line with the lift vector, producing a rolling moment. In the OP’s case, after holding it for a few seconds the lift vector becomes aligned with the CoG again. Once they’re aligned there’s no rolling moment. A force that is radial to the CoG does not produce a moment.

@TomMcW You're right.

The "flies in circles without changing heading after being disturbed" behavior is most likely due to precession of the balance bar.  Disturb the fuselage/rotor shaft, some of that disturbance propagates into the balance bar; once the body has righted, the balance bar continues in a very slightly tilted plane, and the slight righting force from the shaft causes it to precess.

"I cannot picture flight in a circle without changing heading: the helicopter would fly backwards halfway in the circle." - Yes, that's exactly what it does.

The rotor is gyro stabilized. The balance bar is the gyro. If the machine rolls right, the balance bar wants to stay in a level plane and generates a correction by influencing the rotor blades to go where the balance bar wants to be.
The Bell 2 blade teetering rotor system used on the '47 and the Huey used a much smaller version of the same thing, to provide a little bit of inherent stability to the rotor disc, without inhibiting pilot control.

But what about the case where the balance bar isn't level to begin with? If I grab the helicopter and tilt it a little bit, and hold it for a few seconds, the balance bar will tilt to align with the rest of the helicopter. After I let go of the helicopter, what causes the balance bar to return to a horizontal position?

As a gyro, if you force it out of the horizontal plane and hold it for an extended period it will tend to re-align with the new angle but still with a residual tendency to seek level.  Once you let go, the residual tendency to regain level is enough to return the entire rotor to level.

That makes sense, but what I'm confused about is the "residual tendency to seek level". Gyroscopes don't spontaneously level themselves, so what causes this gyroscope to level?

Residual tendency is actually a bad phrase to use.  What is happening is from the fact that the when you let go, the body of the machine, being a pendulum, wants to go straight down, taking the mast with it, and this imparts a tendency of the gyro stabilizer bar to follow the mast back to vertical.  The pendulous mast itself is acting a bit like the self-erecting function of a gyro instrument.

Ah, like in the pendulum fallacy?

"the body of the machine, being a pendulum, wants to go straight down" - But the rotors want to go straight down, too. Gravity never causes an object to rotate (ignoring tidal forces, which are insignificant here). A roly-poly toy is kept upright by the supporting force from the floor, and an airship is kept upright by buoyancy. There must be some aerodynamic force which rights the helicopter—right?

The aerodynamic force is the lift from the rotor.  The body wants to swing under it unless it is accelerating laterally.  Think of a teetering rotor helicopter as a magic disc flying independently with the body as a tennis ball on a string suspended below.  When you fly a Bell machine you are flying the disc around with cyclic, and the machine below is just hanging there going for the ride.  If the rotor is tilted but there is no lateral acceleration, or not enough for the tilt, the machine will try to swing under which will tend to move the stabilizer back to level.

@Koyovis the pendulum fallacy is about the misconception that putting a rigidly attached thrust element at the top of a rocket is stabilizing and at the bottom is destabilizing.  It's the same misconception that makes people think that tail mounted engines on a jet are destabilizing in yaw.  The misconception only applies to a body and thrust element that are rigidly fixed to each other.  A helicopter with a teetering rotor is a free body attached the thrust element by a flexible joint and is basically suspended as if from a rope.  Not the same thing at all.

"The body wants to swing under it unless it is accelerating laterally." That seems like a bit of a vacuous statement, since if the helicopter's attitude is disturbed, that will cause it to accelerate laterally. That seems a lot like saying, "If an airplane banks, then the occupants will feel like they're being pulled towards the lowered wing, unless the airplane turns while it's banked." That's not a false statement... but airplanes do turn while they're banked, so the statement isn't saying very much.

For what it's worth, I feel like we're looking at this question from very different angles. Neither of our angles is wrong, but we're having trouble understanding each other since our perspectives are so different.

You're not understanding how the Bell teetering rotor system works. The body does not have a rigid or semi-rigid connection to the rotor disc like you have with articulated rotor heads where the mast and rotor head are a unit.   On a Bell, or your RC, the rotor disc flies around independently.  If the rotor tilts it doesn't immediately tilt the body. The rotor disc moves sideways and the body swings with it with a tiny bit of lag. Because of the lag, there is an initial displacement of the mast away from perpendicular to the rotor, and the stabilizer bar tries to restore the perpendicularity.


text:
What is the difference between MEL and CDL?
I know the MEL and CDL are the minimum equipment list is provided by the manufacturer, if component missing or damaged means the aircraft must pass the MEL and CDL, I need some suggestions how it was actually working in aircraft maintenance

The MEL lists the limitations of the aircraft when some equipment (autopilot, transponder...) are inoperative.
The CDL deals only with non-structural external parts (wheel fairing...) that may be missing without grounding the aircraft.
From this website, emphasis mine:

The CDL is a listing of regulator-approved non-structural external parts that may be missing but the airplane remains airworthy.
To qualify an item onto the CDL, a restrictive set of conditions must be met, e.g.:

The effect of the missing part upon adjacent structure and systems must be evaluated.
The effect upon airplane performance must be measured.
The combined effect upon the aircraft when more than one CDL item is present must be determined (i.e. the effect of a combination of items missing).

[...]
The CDL should not be confused with the MEL (MEL). While the MEL describes the limitations of aircraft operation in case of a system being inoperative/having malfunctioned (e.g. transponder failure), the CDL deals with situations where external parts of an aircraft are missing/fallen off (e.g. fairings, aerodynamic seals or panels).

FYI, in spite of what Skybrary says, the CDL on my airplane has parts that are not only external (some insulation bags under the floorboards, for instance).

@Lnafziger good to know. It makes sense to be on the CDL though, since insulation bags are neither structural or equipment and the aircraft probably fly just fine (with some limitation) if they are missing.

Thank you for your answer :)..But still, I do not understand the concept of MEL. Please advise me

The practical applications of MELs and CDLs happen when a snag over a defect is raised over something broken or missing, by somebody examining or operating the aircraft.  The exceptions allowed are intended to allow the airplane to dispatch with something inoperative or missing, for some interval of time and possibly subject to some mitigating action by maintenance and/or flight crew.
The objective is to avoid grounding the airplane for repairs right away.  A lot of MEL items allow operation for up to 10 days with a redundant subsystem inoperative for example, usually subject to a maintenance deactivation procedure, or an additional flight crew action, that mitigates the risk of the inop redundant subsystem.
So a typical scenario is a flight crew observes a status message that one of the two channels of some system is inoperative, maybe inbound, or at the gate.  They will consult with the airline's maintenance control organization and look up the subsystem in the MEL and if the subsystem is covered, follow whatever maintenance action (like deactivating an actuator by removing power to it or locking it out) or crew action (like doing an ops test every flight instead of first flight of the day, or at routine check), continue flying, and schedule the repair within the MEL item's time allowance.
When a component has good MEL coverage, a significant reliability problem with it becomes a cost issue, related to the need to do something about it eventually, instead of a causer-of-delays/cancellations, which is a much worse problem.
When an airplane enters service, the OEM makes a huge effort to make its MEL/CDL as expansive and comprehensive as possible, as this has a huge effect on dispatch delays and the overall dispatch reliability of the fleet. They will also grow it over time, and there is normally a permanent working group set up between the OEM and regulator to manage and improve the MEL/CDL over the fleet's operating life.

Nice insights on how those lists affects dispatch and maintenance :)

Thank You for your wonderful and understandable answer.


text:
Why do some U.S. airlines require passenger headrests to be lowered during takeoff?
I've experienced a few times recently where cabin crew have asked passengers to lower their adjustable headrests during takeoff and landing and, in at least one case, it was even part of the pre-recorded safety briefing.
I'm wondering what on Earth would be the reason for this?
I completely understand needing to keep seatbacks and tray tables upright and aisle armrests lowered, but having headrests lowered makes no sense to me. As a tall person, having the headrest lowered is uncomfortable. For extended durations (e.g. a long wait to takeoff, etc.,) it becomes extremely uncomfortable.
Even more concerningly, though, it seems like this would significantly reduce safety in the event of a crash, due to forcing a tall passenger's spine into an unnaturally-forward position and having neither most of the spine nor the head supported. It seems like this would be much more likely to cause head/neck/spine injuries to the passenger during a crash than would allowing passengers to adjust their headrests to properly support their heads and necks (and not push their shoulders forward.)
I see that 14 CFR 121.311(e) requires seatbacks to be in the upright position during takeoff and landing, but I see nothing in the FARs about requiring adjustable headrests to be lowered for takeoff and landing during Part 121 operations, though I may have missed something.
So, my question is what is the reason for this requirement for headrests to be lowered? Is there some regulatory requirement for this or is it an airline-specific thing?

Well, there's some good safety reasons. Probably...

The answer appears to be one of clearing the view of the flight attendants. This FAA circular mentions the requirements that a FA be able to see clearly.

(3)  Direct View.  As used in § 25.785(h)(2) [§ 25.785(h)(1)], "direct view" means direct (line of sight) visual contact with cabin area/main aisle(s), which enables the flight attendant to be made aware of passenger needs relative to safety when the flight attendant is seated with torso restraint (safety belt and shoulder harness) fastened.  Mirrors or other such devices are not acceptable equivalents to direct view, except in those cases where flight attendant proximity to the floor level emergency exit takes precedence over direct view.  Video systems may be an acceptable means of direct view, if the level of conspicuity is equivalent to that provided by line of sight visibility.

This forum thread talks about it with regards to FA regulations

I have been told by many FA's that the reason for the headrests to be down is for FA visibility during takeoff and landing and taxi when they are in their seats. With them up they don't have a clear view of the cabin.

Interesting! +1 It seems surprising to me that the headrest would block significantly more of the FA's vision than the actual head resting on it. It also seems like crash safety of the passengers should take precedence over FA field-of-view. I guess at least the FAs will be able to see the passengers who sustained neck injuries due to having their headrests down? And that FT thread... wow. Reminds me why I don't spend much time on FT (or UA. - haha)

"It seems surprising to me that the headrest would block significantly more of the FA's vision than the actual head resting on it." Exactly my thought, @reirab!

I contacted Delta out of curiosity and this is the reason they gave, as well. Granted, this was a customer service representative, not an airline spokesperson stating an official position, but they said, "For aircrafts [sic] that have headrest that raise and lower flight attendants ask that they be lowered so in the case of an emergency their view is unobstructed."


text:
Can ATC provide VFR flight following to a VFR aircraft in class E airspace?
Just wondering if ATC can do this or not. I'd also appreciate input on whether it's useful or a good idea, and how it looks in the real world.

We already have several questions on flight following that may (mostly) answer this: here, here, here, here

They can. I often pick it up here in the Northeast since the airspace is fairly busy. Generally speaking I was taught that flight following is always a good idea. It is a useful tool to you in the cockpit and can provide advisories of fast moving hard to see aircraft as well as those that may be approaching to overtake you. Generally it helps to improve your radio skills and may reduce your work load ultimately.
As for who to contact, that can change. I had this very question during my training (asked here) and you can find the answer here. If you are departing from a controlled field you can sometimes ask the tower to set it up which they may or may not do. The Airport Facility Directory will have an "approach" and/or "departure" frequency listed for most fields which is generally who you call. If you are in the wrong area they generally send you over to the right controller.

Who do you contact to receive the service in class E? Does it depend on where you are? I'm assuming if the class E (or class E surface) is an extension of a class D airspace, you'd pick it up from the tower. And maybe if you're outside of a TRACON, you'd get it from a center controller?

@jeff0000 I have added a bit to cover that, comment if it does not answer the extension of the question.

Yes, you can request VFR flight following in Class E airspace and 90% of the time you will be flying in Class E airspace.  There’s nothing unusual or difficult about it; you simply request flight following and ATC will usually approve it unless traffic congestion, emergencies, etc consume all available ATC resources for this.

Typically its the different Approaches that provide flight following, not Center, unless you are at higher altitudes.  For example, east coast of MA, you will talk to Boston Approach most of the time. Once you climb higher, like above 5000 ft and start heading westward, they will then hand off to NY Center. And even, you might get handed back to Bradley Approach, Albany Approach, etc. if  you are not up in NY Center's airspace.

Yes, as far as I know the entire US Airspace is covered by one center or another. It is the centers which provide flight following. For example on a recent flight from CYKZ ( Toronto Buttonville ) to KFRG (Farmingdale Republic on Long Island , NY), I asked CYKZ controller for flight following ( on the ground ) and she handed me off to Toronto Center. I again requested flight following and the request was granted. Toronto Center handed me off to Buffalo Center, then to Rochester center, Binghamton Center, Wilkes Barre Center, Boston Center, New York Center and finally to KFRG Tower. I believe if I didn't pick up flight following with Toronto, then I could have picked it up with any of the other centers mentioned above. In the JFK area I have picked up flight following with New York approach ( there are different frequencies depending upon whether you are going East or not). On a recent approach NY approach frequency was very busy and I ended up picking up flight following from Albany approach on my way to Montreal. These rules apply in US only. Canadian rules are different and I am still trying to figure them out.

The real limit to flight following is radar coverage at lower altitudes. If Center can't pick you up on radar, they can't provide flight following. If you're getting flight following and you drop off the scope, you'll hear "radar contact lost, flight following terminated."

*"It is the centers which provide flight following" This is incorrect, for example I can pick up flight following with my local airport and fly 60 or more miles under their radar services before they hand me off to center or another airport. Center does not always provide Flight Following services, just about any ATC radar area with a controller can do it.

ok. My airport - KFRG - is a class D airport and they do have radar coverage but they do not provide flight following as far as I know. OTOH I have always received my flight following from a center along the route. So maybe it is correct to say that all centers provide flight following and some other airports with radar may also provide flight following.

@PrashantSaraswat is correct. The local airport does not provide flight following. However, if the airport is in the coverage area of an approach control then they will hand you off to them. If there is no approach coverage then you are handed off to center. I fly out of KSBP and will get flight following on the ground. I get handed off to Santa Barbara approach and if I stay low I will be handed off to a bunch of different approach controllers all the way to southern California. If I fly north, I will get handed off to Oakland center and don”t get handed off to approach until in the Bay Area.

@Prashant, FRG is a Airport Traffic Control Tower which may receive a radar feed and may provide certain limited radar services. But "flight following" is a service provided by a dedicated radar facility, whether a TRACON or ARTCC. Some ATCTs are co-located with a TRACON and some are not. The radar control facility for FRG (and the rest of the New York area) is the New York TRACON, N90.


text:
How do larger aircraft compensate for reduced visibility from the cockpit, e.g. less visibility of the rear of the aircraft?
In smaller planes, pilots has apparently great visibility in front, on the sides and a good portion of the rear of the plane. But as the plane's size is increased, the visibility is also reduced. One obvious factor is height of the cockpit from ground, but it appears (from pictures) that pilots of bigger Boeing/Airbus planes have too many blind spots. However, I did notice that some bigger planes have a camera behind the nose gear to ease in taxiing etc.
So the question is that how do pilots compensate for this lesser visibility? Do they always need external help (e.g. ground crew)? Is there some sophisticated technology helping them? I am sure they cannot just raise or lower a wing or some other maneuver to check nearby traffic.

nearby traffic (in the air) is warned for by TCAS, on the ground they listen to ground control.

@ratchetfreak TCAS is NOT a replacement for visual acquisition of conflicting traffic. It is a last line of defence against mid air collisions and should not be relied upon for separation.

Blind spot for one pilot may not be blind spot for the other (e.g. the front left sector is a blind spot for the pilot seating on the right but not for the other pilot)

Probably the last thing that a pilot should be worrying about is what's behind him - I mean, y'know, unless pterodactyls suddenly become un-extinct and start grabbing airliners out of the skies. (The movie of this would make "Snakes On A Plane" look kinda lame. OK, well, y'know - lamer... :-)

Appearances can be deceiving. The view "behind" a Piper Cherokee looks something like the image below: no rear window so you can't see what's behind you, but that really doesn't matter since there's no way to go "backward" in this aircraft using the flight controls.

Visibility aside, ground handling is often a a complex production, particularly for larger aircraft or when you have a lot of aircraft in proximity to each other or obstacles.
Moving small aircraft around typically requires at least 3 people. When you're moving forward you will have the person operating the tug and two "wing walkers" who stand a few feet off the wingtips and walk along to make sure the wings aren't going to hit anything. When pushing an aircraft back in a tightly packed hangar there's sometimes a fourth person walking ahead of the tail to make sure you don't back the plane into something.
Since there's not usually much for the pilot to do inside the plane at this point, they usually serve as one of the walkers.
Larger aircraft observe similar procedures, but for push-backs or power-backs it's known that the area behind the aircraft is clear (this is usually coordinated over the radio), and as long as the tug driver or pilot keeps the nosewheel on the appropriate pavement markings there will be wingtip clearance.
Line personnel are also often stationed to guide aircraft to and from parking in close quarters where positioning is critical (e.g. when pulling an airliner up to a gate, or parking a small aircraft on a busy ramp).

The other half of the picture is taxiing under your own power and control.
When taxiing, it's generally assumed that the pilot knows their aircraft's dimensions and will avoid hitting things. This usually works out well, but that is not always the case.
Airport taxiways are marked so an aircraft with a specific maximum wingspan will be clear of any obstacles while taxiing along the centerline, and as you're generally only going forward that's where visibility is most important (you can assume if the area your wingtip was going to hit was clear 15 seconds ago when you last saw it and nothing was heading into that area it's still clear now, and you're relying on the pilot of the aircraft behind you to be paying attention and not taxi into your tail).

I've never had 3 people helping me move a small aircraft (up to a small jet). Even at large airports with busy corporate FBOs. It's virtually always just one lineman, although a few times there has been another person on the ramp keeping an eye on clearance.

On a 152, rear view is like this. It is because it has only two seats, anything bigger doesn't have a nice rear view.

Every time I see that video I have to take a few seconds to remind myself that it's a multi ton mid size jet, and not a toy that's being tossed around by a child.  It's a lucky thing no one was hurt in that incident...

@Farhan my Cessna 210 has 6 seats and a window in the back. I’ve never used it to spot traffic and I don’t think I could even see anything out of it if I tried. I always assumed that the only reason that it was there was to provide light for the passengers—after all we fly in the shade.

Heads-up, the linked video is no longer available.

Large planes generally have quite good visibility. In a document A319/A320/A321 Flight Deck and Systems Briefing for Pilots (can be found in many places on the net, e.g. on slideshare as presentation), or page 16 Airbus shows following diagram of visibility from cockpit:

20 degrees below horizon straight ahead is probably better than most single-engine GA aircraft with large engine cowling in front of the cockpit.

Small aircraft are generally under VFR rules, where they need to have that visibility to look out for other traffic. Larger aircraft usually operate under IFR rules, where (as ratchet pointed out) ATC helps to separate traffic, and TCAS will help as well. At the high speeds at which they fly, visual separation is less useful, anyway.
Also, especially when you start travelling faster, what matters most is what's in front. If everyone makes sure they don't run over someone, you shouldn't have to worry about getting hit from behind.
When at the gate, most aircraft (even the smaller ones) will have a person guiding them in, and during pushback the tug driver has a better view and wing walkers make sure the wing tips clear.
When taxiing, pilots will get a feel for how to control the aircraft from training and experience. Especially in large airplanes, they can be sitting significantly forward of the front landing gear, making it a bit disorienting to maneuver if you are not used to that.
However, incidents happen. You may recall recent incidents where planes have clipped each other while taxiing, such as this one at LAX. Because of these incidents, the NTSB has recommended that improvements such as cameras would help avoid these situations.
The A380 has a camera on its tail that helps the pilots stay aware, but it is unique in this regard (though it doesn't appear to have a wide enough view to include wingtips).

It depends on airspace class, but in most cases ATC only provides separation between IFR flights, but not IFR flights from VFR flights. So if the weather is actually VFR, pilot flying IFR still has to look for and avoid conflicts with VFR flights themselves. And not all flights have at least mode C transponder needed for TCAS to see them.

On the ground, pilots of commercial airliners taxi without the help of ground crews except for entering and departing the gate area.  Once out on the taxiways they follow the painted line.  Keeping the nosewheel on the line pretty much guarantees that there is enough clearance for the plane including the wingspan.

More of a historical anecdote, but the British VC-10 had a periscope (!) with no less than four different mounting locations- two in aft cabin (for viewing each side the tailplane), the cockpit (originally for the sextant) and the electronic bay looking downwards to inspect the landing gear. It appears to be more for technical reasons rather than operational ones.

Photo: J. Hieminga

"Photo BOAC via C. Messenger"

Normally the periscope will be stored in its box somewhere in the back of the aircraft. In A4O-AB, the Sultan of Oman's aircraft, this box is kept in the aft galley. The periscope itself is about two and a half feet in length, and is a pretty sturdy piece of ironmongery. There are two mountings for the periscope in the roof of the fuselage, which are similar in design, both to be found just in front of the rearmost toilets in the aircraft. These can be revealed by undoing two quick-release catches on one of two small triangular panels in the roof. This provides the sight below.
In its installed position the periscope can be rotated 360 degrees, and the control on the side tilts the mirror at the top about 60 degrees up and down. All together you can look all the way around the aircraft, except for straight up at the sky (but there shouldn't be much to look at up there anyway). By switching from one mounting to the other both sides of the fin can be viewed. From the lefthand mounting, looking to the right wingtip will have you looking at the base of the fin as the periscope sticks up right next to it.
This same periscope can also be mounted in two other locations. The first of these is in the cockpit roof. The mounting installed here was meant originally for use with a periscopic sextant, but will also take the periscope. During BOAC crew training flights where Dutch Roll characteristics were demonstrated trainees were sometimes given a chance to view the back end of the plane from here to see the effects. The second location is located in the bottom of the fuselage and was included to enable the crew to check the status of the landing gear if any uncertainty about its position was present amongst the crew. It consisted of an opening in the electronics bay access hatch located underneath the fuselage. Unlike the others there was no pressure shutter to cover this opening - it was just a plug that could be removed and replaced with the periscope - since it would generally only be used at lower altitudes where the pressure differential would be lower. Still it has been confirmed to me by an ex-BOAC First Officer that the plug could be removed when flying at 39000 feet, although this needed quite a pull and the result was quite noisy!

Source of the quote and pictures: VC10.net

Since you brought it up, “Lindbergh had direct vision only from the side windows, relying on a periscope to see straight ahead.” when he made his solo Atlantic crossing in the Spirit of St Louis,  https://www.britannica.com/topic/Spirit-of-Saint-Louis

authors: Bob Jarvis - Слава Україні


text:
Why are different routes flown between the the same pair of airports?
On two of my trips (same airline, same kind of plane 777), I noticed that the airplane took very different routes (from New York to UAE).
1st Trip in 2012 (blue): Duration 11h 30m
2nd Trip in 2013 (green): Duration 13h 00m
I know that the blue line is the shorter distance, and probably also because of jet streams, it took lesser time. But why a plane would take this green route, even not considering jet streams?
Since I am not an important person, I could not ask the captain of the plane this question at the end of journey.
P.S.: I observed the flight paths on the entertainment system this airline has for all passengers.

Since I am not an important person, I could not ask the captain of the plane this question at the end of journey - Actually, I don't think a pilot would mind you asking them this.

That might be correct, but approaching the pilot would be hard.

@DannyBeckett They might in the UAE....

@Lnafziger thats when you find the expat FO and bug him instead

@casey "bug" him... "BUG" him. I read that as "mug" him, and though that was and interesting way of extracting flight information...

How is the blue line shorter than the green line? The green seems far more direct.

It can be a matter of jetstreams, weather or that the pilot wanted to avoid part of the airspace due to sector taxes and/or restriction. @MarkyMark that is what it may seem, but you must account that the Earth is not a plane and orthodromic routes are shorter than lexodromic ones.

@MarkyMark It might look more straight, but since the earth is spherical, the blue line becomes shorter.

@MarkMicallef That might be a good trick question to flat earth believers.

Lots of reasons are possible, ranging from winds to other issues. In this case I'm willing to bet that the first flight was based on the most efficient routing to take advantage of tailwinds en route and that the second was avoiding Syrian and Turkish air space because of the political issues there earlier this year.
See also:

Flight Delays Incurred Avoiding Syrian Airspace
Royal Jordanian stops flying over Syrian airspace

Thanks. Political reasons might be a cause but the second trip (green) was in last month, and on my way back, the plane followed the blue route.

Regardless, I think the most efficient route may not be necessarily be fastest.

Additionally to the answer of user241, countries have different route charges that may influence the airlines in their cost trade-off to take different routes. Route charges may be negotiated between airlines and countries, leading to different routes by different airlines.


text:
Do aircraft manufacturers produce their own black boxes?
Do the manufacturers who build airplanes use their own "black boxes" in the planes, or are there companies whose sole purpose is to develop black boxes?
For example, in 2010 in the Airblue Flight 202 incident, the black box had to be sent to France for data recovery:

He stated that the box would be examined by "foreign experts" in
  Germany or France as Pakistan does not possess the equipment to decode
  the flight recorders. He also stated that the process of extracting
  information may take six months to a year. The Pakistani
  authorities decided to send the CVR and FDR to the Bureau d'Enquêtes
  et d'Analyses pour la Sécurité de l'Aviation Civile (BEA) in
  France.

It is not clear who developed that black box. Was it the same company that manufactured the airplane?
Also, can the buying airline use their own black box or their own security protocols for data encryption or encoding, to avoid having to send the black box out for analysis?

Your text says Germany, your quote France O_o

There's seldom a reason for manufacturers to design and build their own black boxes. It's expensive to run a production line to design, build a test a few. It's just cheaper to get an agreement with a company to make them for you. The second photo below shows a popular recurring model from Honeywell, which appears to have been used on the A330, B737 and B777.
Opening the black box can be a sensitive operation, especially if there's the fear that the chips might have been damaged. It's better to send it to somebody who's done it before rather than trying to master it on the first attempt, especially if you don't have the resources. Airplanes accidents don't happen very often.
Note that the case you're referring to, the Aircraft also appears to have been built by Airbus in Hamburg, and hence the German Accident Board automatically gets involved as well, much like how the NTSB gets involved in all Boeing crashes.

(source: naij.com)
A smashed up flight data recorder.

(source: dailymail.co.uk)
A photo from the NTSB with the Asiana flight 214 recorders
Another reason for this is if there might be political motives behind the crash, it's preferred to send it to a third party for analysis which is less susceptible to bias or interference.
If I gather correctly, data on the flight data recorder won't be encrypted at all- it will be fed out as serial data, but you will need the devices and software to understand this information, and if the outside connection is broken, you'll need to work your way to the chips themselves.

(source: com.au)
Photo showing apparently how black box are 'read'.
Furthermore, sending it to the company is not always a good idea- they might have motives to change the truth, especially if it was caused by dodgy maintenance or bad training, nor to the manufacturer, if it was production error. The accident investigation board is the best people for this matter.

so you are implying that the Plane's manufacturers do not develops their own and there are various investigation boards whose purpose is to recover the data and who have the expertise?

@SufiyanGhori I can't seem to find evidence for any large aircraft manufacturers that build their own.

@SufiyanGhori Accident investigation boards are (hopefully) non-biased and experts at this sort of thing and normally always do the readout (or supervise it at least i think), but sometimes it's better to send it to a larger group of accident investigators if you're own department is only a small one.

Wonderful answer indeed, but could the buying party develop their own Black Box and replace them with the Vendor's installed ?

@SufiyanGhori The criteria for black boxes is very strict and extensive testing is required to certify them, such as fire, water and impact testing . There are a lot of specialized materials inside them like fireproof covering and shock-proof electronics, that need special skills to design and build, so even Boeing will get somebody else to do it for them, rather than having to hire people and do it themselves, especially if there are companies (such as honeywell) who have know-how and build for everybody, but if they really wanted to, I see no reason why they could not build their own.

@SufiyanGhori Boeing and Airbus even get their landing gear built by specialized companies who have the specialized knowledge and tools abroad, as for the engines.

@Manfred Re: your last paragraph, the investigative agencies take the protocol surrounding the "black box" very seriously for precisely this reason. The NTSB has even gone so far as to kick an airline out of the investigative process because they accessed the Flight Data Recorder before turning it over to the NTSB for analysis.

@voretaq7 Good link :)

Airbus headquarters are in Toulouse, France and therefore French BEA is considered representative of state of manufacture for Airbus aircraft (and all ATR aircraft as ATR has headquarters next to Airbus). The page about the accident in question does say it was eventually BEA who analysed the black boxes.